A mobile cellular telephony terminal allows it user, as he or she travels, to overcome any constraint in receiving or transmitting a telephone call by radio. From time to time, the terminal moves from one radio coverage cell to another adjacent one, and on each occasion it can connect to the base radio station providing radio coverage for the cell in question.
This complete freedom of movement does, however, have the drawback of ruling out any accurate location of an individual in distress, who would be incapable of indicating his or her exact position, while a wirebased telephone network makes such location possible. Although a base radio station can actually indicate to the cellular network operator the presence of a given subscriber in its cell, its area is too wide for this information to be useful.
In order to solve the problem mentioned above, the Applicant Company has contemplated terminal location by triangulation with respect to the base station of the cell and the stations of the neighboring cells, the transmissions from which can be received by the terminal. Measurements of distances to the stations by measuring the time of flight of the waves and/or measurements of azimuthal angles of the reception directions with respect to the stations, the positions of which are known, would in theory make it possible for the terminal to determine its relative position with respect to these known positions and to determine its absolute position therefrom, in order to indicate it to the station of the cell in which it is located. In practice, however, by interfering with the propagation of the radio waves, reflection or diffraction by a variety of obstacles would greatly distort the measurements by which the reception direction is determined, while multiple radio-wave paths would lead to a temporal spread of the theoretical reception time of a wave output by a station and would create an excessive uncertainty on the distance measurement taken on the basis of measuring the time of flight of the radio wave. A satellite radio-positioning receiver, such as GPS (Global Positioning System) would encounter the same problem. In an urban area, large buildings often generate errors of this type, and the solution of determining position by triangulation must therefore be ruled out.
The Applicant Company has also envisaged navigation by reckoning, which is used for vehicles, in which the distance covered at any time is measured using an odometer and, by composition with the heading taken by a magnetometer, a displacement vector used to update a previously calculated position is calculated as travel progresses. The odometer may be replaced by a Doppler radar or a camera which, pointing at the ground, measures the velocity of the image that is recorded. It is not, however, possible to impose a permanent constraint of this type on a pedestrian carrying the terminal, and, in practice, the requisite sensors mentioned above need to be fitted on a vehicle.
Consideration may also be given to inertial navigation, currently used on board aircraft, which affords the carrier of the terminal complete freedom of movement and attitude. Using a triaxial accelerometer and a triaxial gyroscope, an inertial unit calculates the instantaneous velocity vector of the craft in question by integration with respect to time, and an additional integration with respect to time provides the displacement with respect to a defined initial position on the basis of it. However, it is necessary to be able to update cyclically the position which is supplied by the unit and on which the position calculations depend, in order to limit the error which would otherwise increase indefinitely over time.
It is the latter technique of position determination which the Applicant Company has chosen in order to solve the problem of locating a telephony terminal, using means which do not have the above drawbacks and which supply position indications having a limited error.